1. Field of the Invention
The present invention relates to an apparatus and method for zeroing, calibrating and setting a surgical knife.
2. Description of Related Art
There presently exist a surgical procedure for correcting defects in the human eye such as myopia and astigmatism, which is commonly known as radial keratotomy. Radial keratotomy includes cutting a series of radial, non-penetrating incisions on the outer periphery of the cornea. The incisions alter the shape of the cornea and correct the deficiencies in the eye.
The incisions are typically performed by a knife that is held and manipulated by the surgeon. Surgical knives used for radial keratotomy have a blade which can move relative to a footplate that extends from the body of the knife. The incisions are formed by placing the footplate on the surface of the cornea and moving the knife across a portion of the eye. The depth of the cut is therefore determined by the relative distance between the footplate and the tip of the blade.
The knife typically has a thimble that allow the surgeon to move the blade relative to the footplate. The length of the blade is set by rotating the knife a predetermined number of turns. A typical radial incision depth is between 580-620 microns with a required accuracy of plus or minus 5 microns. It is therefore important to establish a high degree of accuracy when setting the blade of the knife.
U.S. Pat. No. 4,750,489 issued to Berkman et al, discloses a surgical knife and a method for setting the blade relative to the footplate of the knife. Incorporated into the body of the Berkman knife is a linear variable differential transformer ("LVDT") which senses the position of the blade relative to the footplate. The LVDT is coupled to a visual display which provides a readout that indicates the distance between the tip of the blade and the footplate.
To obtain an accurate readout, the blade must be initially aligned or "zeroed" with the footplate. Zeroing the Berkman system is performed by inserting the knife into a zeroing sleeve. The zeroing sleeve has a reflective membrane that reflects a pattern of light to a plurality of light sensors. The knife is inserted into the sleeve until the footplate is in contact with the reflective membrane. The knife is then adjusted until the blade advances to a point where the tip deflects the reflective membrane. Deflection of the membrane disturbs the light pattern which is sensed by the sensors and interpreted by the visual display as the zero position of the blade. The Berkman system for calibrating the knife has been found to be somewhat unreliable to use and expensive to produce.
U.S. Pat. No. 4,662,075 issued to Mastel et al, discloses another method of zeroing and setting a surgical knife that uses an apparatus typically referred to as a micron table. The micron table has a microscope coupled to an X-Y table. The lens of the microscope has a cross shaped reticle which provides a pair of reference lines for the user. The knife is placed on the X-Y table and the table is adjusted until the footplate and blade are centered within the reticle. The knife thimble is then rotated until the tip of the blade is zeroed with the footplate of the knife. After the blade is zeroed, the X-Y table is adjusted to move the footplate and blade a predetermined distance from the center of the reticle. The knife thimble is again rotated so that the blade is moved back to the center of the reticle. The knife is then transferred to a sterile operating field.
During the transportation of the knife, the tip may move to a different location, resulting in an inaccurate blade setting. When this occurs the knife must be returned to the micron table and reset. Additionally, while performing surgery it may become apparent to the surgeon that the blade setting is either too short or too long. Resetting the knife again requires a trip back to the micron table. The micron table is typically located separate from the operating field, wherein the knife may require resterilization every time the knife is calibrated and set. Such a procedure wastes valuable operating time. It would therefore be desirable to have a system for accurately zeroing and setting a surgical knife at the operating site.